Although audio signals are often transmitted in digital form, analog transmission remains attractive for many applications, particularly where bandwidth and dynamic range constraints of the transmission channel limit the potential data rate of digital transmission. Audio encoding schemes have been developed that permit audio transmission at lower data rates, but the data rate reduction is typically accompanied by various drawbacks. These include digital signal processing complexity, degraded audio quality, encoding and decoding delays, and abrupt performance degradation with weakening signals.
Conventional analog transmission techniques can efficiently convey the frequency spectrum of an audio signal without the excess bandwidth of high digital data rates or the disadvantages associated with data rate reduction. Such techniques require strong signals to preserve high audio dynamic range, however, which is ultimately limited by noise in the analog transmission circuitry. This problem is often mitigated by “companding” the signal.
Companding involves compressing an audio signal by variably amplifying it depending on signal level (with stronger signals being amplified less than weaker signals), transmitting it over an analog channel, then expanding the audio signal at the receiving end of the channel by subjecting it to a complementary variable amplification. The two variable amplifications complement each other so that expansion restores the final signal to its original amplitude. The compressed audio signal requires less dynamic range than the original for faithful transmission over the analog channel. However, companding requires compromises in selecting the attack and release times used in tracking amplitude variations. The compressor should track variations rapidly enough to compress a signal effectively but slowly enough to avoid distorting its low-frequency components. The resulting design compromise attempts to balance compandor performance with compandor artifacts like signal distortion and “pumping” and “breathing” sounds that many listeners find equally objectionable.
Dual-band compandors have been developed in an attempt to alleviate these audio problems. By separating an audio signal into high and low frequency bands, a dual-band compandor can process each band with attack and release times better suited for the frequencies in question. But the selections made for each band are still compromises, and compandor artifacts and signal distortion can remain problematic. In addition, the expansion stage of a multi-band compandor is difficult to implement accurately.
Accordingly, a need remains for a method of transmitting audio signals over an analog channel with the dynamic range benefits of companding but without significant audio degradation of the type conventionally associated with companding, and without the difficulty of multiple band companding.